1. Field of the Invention
This invention relates to wireless communication methods and system. More particularly, the invention relates to a method and system for optimally serving stations on Wireless LANs using a Controlled Contention/Resource Reservation protocol of the IEEE 802.11e standard.
2. Description of the Prior Art
IEEE 802.11 is a standards body developing Wireless Local Area Network (WLAN) Standards [802.11, 802.11a, 802.11b]. Recently, that body has started development of a supplement that would specify the support of Quality of Service (QoS) within 802.11 WLANs. This work is being carried out by the 802.11e Task Group, and the most current draft of the QoS extensions being developed (as of the writing of this application) can be found in [802.11e]. A set of protocols has been proposed for use in 802.11(e) based on centralized control of the wireless media. In this protocol set, during specified periods of time called Contention Free Periods (CFPs) and Contention Free Bursts (CFBs), STATIONs (STA) may only use the wireless medium when granted permission from the Hybrid Coordinator (HC). The HC is responsible for allocating bandwidth on the wireless medium and ensuring that QoS needs are met. The HC generally grants the use of the medium to a STA by polling it. This transfers control of the medium to that STA for a limited period of time. Control of the medium must then revert to the HC.
A problem which is addressed within the 802.11e draft is how to make the HC aware of changing bandwidth needs for the STA it serves. A protocol included in 802.11e for doing this was first proposed in [00/33] to the 802.11 community. The protocol is termed the Contention Control/Resource Reservation (CC/RR) protocol. In this protocol, the HC grants the medium for use by Resource Reservation (RR) frames by transmitting a Contention Control (CC) frame. Only RR frames may be transmitted during the time period specified by the CC frame. This time period is called the Controlled Contention Interval (CCI). The RR frames detail the bandwidth needs of the STA transmitting them. Several parameters for the CCI are specified by the CC frame. These include a Permission Probability (PP), the number of Controlled Contention Opportunities (CC_OPs), and a set of flags indicating which Traffic Categories (TC or Priorities) may compete for the medium with RR frames during an upcoming CCI. The protocol states that when a STA receives a CC message and wishes to send a RR for an appropriate TC, it will chose a random number between 0 and 1. If the random number is less than or equal to the PP value, the STAs is permitted to transmit the RR. It then randomly selects a CC_OP in which to transmit (Note: the current draft of 802.11.e has eliminated the PP value, so STAs transmit a RR during a CCI whenever a permitted TC has one to transmit). Since other STAs may be transmitting RR frames, there is a possibility that multiple RRs will be transmitted in a CC_Op, and none will be received correctly (though it is possible due to RF capture effects one will be correctly received despite the contention). Such a CC_OP would be considered collided or “busy”. If only one RR is transmitted in the CC_Op, it most likely will be received correctly (although, due to interference or noise on the wireless medium or propagation issues it is possible, it will be lost anyway). And finally, some CC_OPs will not contain any RR, and in some sense those “empty” CC_OPs waste bandwidth on the medium.
While [00/331 and 802.11e detail the overall protocol, required frame formats, and how the transmitted CC parameters are used by the STA, there is no detail on how the key parameters are determined and set by the HC. What is needed in the art are methods which advantageously set the parameters for the CC/RR protocol so as to optimize performance for efficient use of the medium.